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T. Buckup et al.
signal depends (i) on the population difference squared between the involved states
((n 2 = (n e −n g ) 2 ) and (ii) on the intensity of each incoming beam (I pu I St I pr ∼ I 3 ).
By decreasing the energy of the total DFWM sequence, the DFWM signal is automatically diminished. This overall decrease of the DFWM can be compensated by
increasing the energy of the initial pump pulse, which also leads easily to a suppression of the ground state contributions. This can be easily understood since the
population difference between the excited state and ground state diminishes.
9.3 Results and Discussion
In the following, three different applications will be presented. Firstly, pure DFWM
is applied to RPSB in order to disentangle ground from excited state vibrational
coherence. This first experiment is followed then by two experiments using pump
DFWM applied to carotenoids, where the excited dynamics is exclusively investigated.
9.3.1 Assignment of Vibrational Coherence to Electronic States
Using Pure DFWM
9.3.1.1 Introduction
Oscillations in the DFWM signal contains important information about vibrational
coherence dynamics. In this context, a major issue of the signal analysis is the assignment of such modulations to specific electronic states of the sample molecule,
since resonant excitation generates vibrational coherences in both ground as well
as excited electronic states. In order to achieve this goal, spectral tuning of the excitation spectra together with spectral resolution of the signal light in DFWM experiments have previously been used for investigations on coherence dynamics in
Bacteriorhodopsin (BR) and retinal protonated Schiff-bases (RPSB) [29, 30].
Particular interest resides in the characterization of wave packet dynamics in
these samples because of the occurring photo-isomerization reactions which are important model reactions in terms of their biological relevance [3] as well as technical applications as photo-chemical switches [34]. The observed vibrational dynamics embrace a large energetic region ranging from around 100 cm −1 to more than
1500 cm −1 . High-frequency modes (> 800 cm −1 ) reflect single- and double-bond
stretching or substituent wagging motion where the vibrational frequency is dependent on the specific structure of the chromophore and its environment. Contrary to
that, low-frequency modes (< 800 cm −1 ) represent delocalized motion from groups
of atoms. By following the evolution of the vibrational frequencies accompanying,
e.g. photo-induced chemical reactions, one may hope to directly identify the chemically relevant modes [14, 35]. In this context, ground state vibrational modes which
T. Buckup et al.
signal depends (i) on the population difference squared between the involved states
((n 2 = (n e −n g ) 2 ) and (ii) on the intensity of each incoming beam (I pu I St I pr ∼ I 3 ).
By decreasing the energy of the total DFWM sequence, the DFWM signal is automatically diminished. This overall decrease of the DFWM can be compensated by
increasing the energy of the initial pump pulse, which also leads easily to a suppression of the ground state contributions. This can be easily understood since the
population difference between the excited state and ground state diminishes.
9.3 Results and Discussion
In the following, three different applications will be presented. Firstly, pure DFWM
is applied to RPSB in order to disentangle ground from excited state vibrational
coherence. This first experiment is followed then by two experiments using pump
DFWM applied to carotenoids, where the excited dynamics is exclusively investigated.
9.3.1 Assignment of Vibrational Coherence to Electronic States
Using Pure DFWM
9.3.1.1 Introduction
Oscillations in the DFWM signal contains important information about vibrational
coherence dynamics. In this context, a major issue of the signal analysis is the assignment of such modulations to specific electronic states of the sample molecule,
since resonant excitation generates vibrational coherences in both ground as well
as excited electronic states. In order to achieve this goal, spectral tuning of the excitation spectra together with spectral resolution of the signal light in DFWM experiments have previously been used for investigations on coherence dynamics in
Bacteriorhodopsin (BR) and retinal protonated Schiff-bases (RPSB) [29, 30].
Particular interest resides in the characterization of wave packet dynamics in
these samples because of the occurring photo-isomerization reactions which are important model reactions in terms of their biological relevance [3] as well as technical applications as photo-chemical switches [34]. The observed vibrational dynamics embrace a large energetic region ranging from around 100 cm −1 to more than
1500 cm −1 . High-frequency modes (> 800 cm −1 ) reflect single- and double-bond
stretching or substituent wagging motion where the vibrational frequency is dependent on the specific structure of the chromophore and its environment. Contrary to
that, low-frequency modes (< 800 cm −1 ) represent delocalized motion from groups
of atoms. By following the evolution of the vibrational frequencies accompanying,
e.g. photo-induced chemical reactions, one may hope to directly identify the chemically relevant modes [14, 35]. In this context, ground state vibrational modes which